Answer the MethodContext to which the receiver refers
(i.e. the receiver itself)

isBlock

Answer whether the receiver is a block context

isDisabled

Answers whether the receiver has actually ended execution and will
be skipped when doing a return. BlockContexts are removed from the
chain whenever a non-local return is done, but MethodContexts need to
stay there in case there is a non-local return from the #ensure:
block.

isEnvironment

To create a valid execution environment for the interpreter even
before it starts, GST creates a fake context which invokes a special
``termination'' method. Such a context can be used as a marker for
the current execution environment. Answer whether the receiver is
that kind of context.

isUnwind

Answers whether the context must continue execution even after a
non-local return (a return from the enclosing method of a block, or
a call to the #continue: method of ContextPart). Such contexts are
created only by #ensure:.

mark

To create a valid execution environment for the interpreter even
before it starts, GST creates a fake context which invokes a special
``termination'' method. A similar context is created by
#valueWithUnwind, by using this method.

Answer whether the receiver is a context that should be hidden to the user
when presenting a backtrace. Such contexts are identified through the
#exceptionHandlingInternal: attribute: if there is such a context in
the backtrace, all those above it are marked as internal.
That is, the attribute being set to true means that the context and all those
above it are to be hidden, while the attribute being set to false means
that the contexts above it must be hidden, but not the context itself.